1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to new and improved methods and apparatus for elevator systems whose drive machines include a DC motor powered by a dual converter power supply.
2. Description of the Prior Art
Elevator systems of the traction type include an elevator car connected to a counterweight via a plurality of steel ropes reeved over a drive or traction sheave. The drive sheave is commonly driven by a DC motor whose power source is a solid state dual converter. The dual converter includes two converters, each of which includes a plurality of controlled rectifier devices, connected and gated to exchange electrical energy between alternating and direct current circuits. One converter is connected such that when operative it provides armature current in one direction, and the other converter is connected such that, when operative, it provides armature current in the opposite direction. An error or reference control signal developed in response to the actual performance of the elevator system versus the desired response, selects which converter bank should be operative, and the magnitude of the armature current to be supplied by the operative converter.
It is common during the operation of the elevator system for the error signal to require the torque output of the drive motor to be quickly reversed. Converter bank switching is accomplished by retarding the firing angle of the gate drive pulses applied to the operative converter to a limit called the inversion end stop, to insure that current is extinguished in the operative converter bank. When current is extinguished, the other converter bank is enabled and the firing angle of the gate drive pulses applied to this converter is advanced towards rectification to develop armature current from the oncoming converter bank.
When torque must be quickly reversed, it is important that bank switching be accomplished as quickly as possible, to reduce the "dead time" during which the converter is not following the error or reference signal. Thus, in order to speed up the process of moving the firing angle back towards rectification, a bank switching "pull-through" bias is injected into the current control loopfrom the time the new converter bank is enabled until the start of current flow from the new converter bank.
While the injection of the "pull-through" bias during bank switching helps to speed up the bank switching process, it also presents a problem under balanced load conditions, i.e., when the weight of the elevator car and its load is close to the weight of the counterweight. When the elevator car carries a balanced load at constant speed, the armature current is close to zero. Only small changes in current are required to overcome disturbances caused by areas of higher or lower than normal friction in the hoistway, or to overcome slight imbalances in compensation. Because the "pull-through" bias causes the firing angle to advance by larger than normal steps, there is a tendency to overstep the required firing angle when the current to be supplied by the new bank is close to zero. When this happens, a "bump" of current of 5-10 amperes may occur as conduction begins. This "bump" tends to set off oscillations in the highly resonant elevator system, commonly called bank-switching jitter. The current "bump" also tends to accelerate the elevator car more than the desired amount, resulting in an immediate need to decelerate the car by switching back to the other converter bank. The process may then repeat again. If the current bumps continue, the oscillations in the elevator car may build up to the point where the ride quality is deleteriously affected.